1. Field of the Invention
This invention relates to a refrigerant for producing ice. In particular, the invention relates to a hardly-water-soluble refrigerant for heat storage in ice (ice heat storage) of direct contact type, i.e., freezing water by bringing the refrigerant into direct contact with water and evaporating the refrigerant, which refrigerant is highly resistant to combustion and has a high chemical stability.
2. Description of the Prior Art
Recently, annual peak demand of electric power in utility systems depends on cooling load. To lower the height of such peak demand, attention has been paid to cooling system with ice heat storage by using off-peak night power, and construction of actual ice heat storage systems has been started for the purpose of cooling of buildings and industrial processes.
However, the use of such ice heat storage system has not spread so extensively, because of the present level of its cost and performance. To expand its applications, the so-called direct-contact-ice-making system has been proposed; i.e., an ice making system using such a refrigerating cycle in which a refrigerant is brought into direct contact with water to make ice. Thus, one can expect not only improvement in operational efficiency due to more efficient heat exchange through direct contact of the refrigerant with water but also cost reduction due to elimination of heat exchanger for ice making.
Various types of direct-contact-ice-making systems have been proposed, but the following conditions are commonly required to refrigerant regardless of the difference in the type of system.
(a) Insoluble to water (hardly-water-soluble)
(b) Free from hydrolysis and chemical stability even after prolonged use
(c) Free from clathration (not to form clathrate compound of refrigerant in water, or hydrate of refrigerant)
The above condition (a) is necessary to evaporate the refrigerant for fulfilling the function of the refrigerant, and if dissolved in water, it becomes difficult for the refrigerant to evaporate. The above condition (b) is necessary to use the refrigerant over a long period of time as sealed in the ice making vessel including a series of devices for refrigerating cycle. The above condition (c) is necessary, because if transformed into clathrate, refrigerant molecules lose their activity due to their inclusion in cavities of the crystal lattice of water, and hence, in order to maintain sufficient amount of active refrigerant molecules in an ice making vessel for ensuring the desired ice making ability, the amount of the refrigerant to be sealed in the ice making vessel becomes very large (10 to 30 weight percent of water therein).
Most conventional direct-contact-ice-making systems use either one of the following compounds as refrigerants satisfying the above conditions.
(1) R114 (Dichlorotetrafluoroethane)
(2) RC318 (Octafluorocyclobutane)
(3) Pentane
However, the above (1) R114 has a shortcoming in that it slightly hydrolyzes and produces hydrochloric acid and the like, so that the ice making vessel is required to be made of stainless steel, resulting in a cost increase. Further, it has a risk of destroying the stratospheric ozone layer, and hence it cannot be used at the present from the standpoint of environmental protection.
The above refrigerant (2) RC318 does not contain any chlorine and free from the risk of destroying the stratospheric ozone layer. Its saturation pressure for room temperature is, however, higher than that of R114, and it requires a stronger heat storage tank as compared with R114, resulting in a cost increase. More specifically, its saturation pressure for 0.degree. C., when co-existing with ice, is 1.3 kg/cm.sup.2 (128 kPa) in terms of absolute pressure, but its saturation pressure for room temperature, e.g., 25.degree. C., increases to 3.2 kg/cm.sup.2 (314 kPa) in terms of absolute pressure. Thus, it becomes necessary to provide various reinforcements in the heat storage tank, both during construction and during operation, so as to meet safety requirements of various laws and regulations, such as "Safety Code for Pressure Tank". Since the heat storage tank must have a volume proportional to the desired magnitude of heat storage, such tank is one of the major factors affecting or increasing the cost of direct-contact-ice-making system. Hence, consideration for reducing the cost of heat storage tank is extremely important from the view point of expanding the use of direct-contact-ice-making system.
Pentane of the above refrigerant (3) is free from the risk of destroying the ozone layer, does not hydrolyze, has a low saturation pressure (below atmospheric pressure or negative relative thereto) for room temperature, and has a boiling point of 36.degree. C. for atmospheric pressure. Thus, the cost of heat storage tank in case pentane refrigerant is low. However, pentane is highly combustible and safety precaution must be taken. Besides, pentane can be subject to biological activation and may be decomposed by such activation; e.g., it can become food of microorganism such as anaerobic bacteria and the like.
An object of the invention is to provide a hardly-water-soluble refrigerant for producing ice, which is substantially incombustible and free from risk of destroying the stratospheric ozone layer and evaporates while in contact with water.